4. Representing Time

The simultaneous representation of the spatial and temporal
components of archaeological and historical information has always posed a
problem, because of limitations of the paper medium. The commonest approach –
exemplified by popular atlases such as The Atlas of World Archaeology
(Bahn 2000) is the
representation of time slices (generally determined by cultural periodisation)
through separate maps or distinctive symbologies on a single map. Where greater
detail is known, or a process can be inferred between two known endpoints (such
as the expansion of an empire or troop movements in a battle), trends through
time are often illustrated through the use of arrows or successive fronts. Space and time are often represented together as a space-time 'cube' (Figure 1):

Figure 1: The space-time cube, showing evolution of a region through time

Perspective views of the space-time cube, such as Figure 1, are pretty much
limited to displaying maps at well-separated points in time (a horizontal plane)
or sites with a known time range (a vertical line). Combining time with all
three spatial dimensions in a perspective view is pretty much impossible.

Computer-based methods

Although computer screens are no more dimensional than paper, they offer us several methods for overcoming the shortcomings of two-dimensional representation, resulting in part from their ability to calculate
perspective but mainly from their ability to generate new versions rapidly on demand.

In order to represent continuous processes, such as spread
of cultural traits or changing environments, one ideally needs to be able to set
up a true volumetric view of the space-time cube. A volumetric view would allow
us to understand the internal structure through the use of transparency, density
and fog effects, rotation of viewpoint and/or stereo viewing. Such technology is
currently beyond the financial and technical reach of most archaeologists.
However, some simpler techniques can go a long way (see also Johnson 1999, 2.6)

Interactive maps

Interactive maps allow one to slice
the space-time cube horizontally (as shown in the animation in Figure 1), viewing a map of the area of interest at any given time and
changing the time – along with other aspects such as zoom and symbolisatin at will. The map is regenerated immediately, allowing the observer rapidly to observe changes (most commonly by dragging a slider bar or clicking
a forward/backwards button). Interactive maps allow the viewer to ask
questions, revisit views of the map, turn layers on or off, zoom in or out,
drill down for greater detail or experiment with symbolisation. As such they are
suitable for enquiry and in a museum or educational context will probably be
used in conjunction with pre-determined views, contextualising instructions or
goal-setting questions. They may also be used to present large amounts of data
for free exploration and research. We used this approach in the Sydney TimeMap
kiosk discussed in section 6.1.

Map animation

Map animation[1] (Campbell and Egbert 1990; diBiase 1994; see for example Figure 1 and several examples in Johnson and North 1997)
builds on interactive mapping by linking a large number of time-slice maps into an animation, with or without transition effects between frames, using 'desktop time' to represent historical
time. This approach typically generates very large non-interactive animation files (movies) such as the Mongol Empire animation illustrated in Figure 2. Map animations provide a more passive experience suited to
highly curated visitor experiences or as resources within a structured curriculum, as well as providing a means of making simple points in an
engaging, easily assimilated form. Our animation of the Mongol Empire shown in Figure 2 illustrates several things which might not be obvious in a
series of static maps:

the rapid pace of conquests separated by long periods of stability – history
is not uniform;

the restriction of longer term occupation to the 'steppic' zone, the natural habitat of these nomadic people;

the
short-lived nature of their incursions into different environments (e.g.
South-east Asia) and/or where communication routes are stretched (e.g.
Europe).

Temporal interpolation

Temporal interpolation uses assumptions about the
trajectory of change between recorded data points to generate best-guess
intermediate maps or animation frames (Johnson 1999, 2.4). One common method is
to place recorded versions of geographic objects along a timeline in an
animation package such as Flash, and allow the package's shape-tweening
algorithm to generate representations for the intermediate frames. This tweening
capability is built into Flash player, resulting in much smaller movies - our
Flash animation of the Shilla Dynasty, Korea (Figure 3), which is of far higher
quality than the Mongol animation shown in Figure 2, uses only 1.3 MBytes for a
10-minute animation (approximately two orders of magnitude more compact). More sophisticated animation would determine the
interpolation by modelling or simulating the processes behind the changes (e.g.
Spikins 1997; Kohler and Carr 1997), thus conforming more accurately with reality (assuming the model used has some predictive
power).

Interactive map animations

Interactive map animations which allow the user to interact
with an animation, turn layers on or off and link from objects on the map
to additional information, can be constructed with animation products such as
Flash. An excellent example of this approach is the FlashGIS toolkit
(Jessee 2001) - see for example Salem
witch trial accusations and Civil
War engagements. One of the particularly nice features of
these interactive animated maps is the way the time bar indicates distribution
of events and varies the animation rate according to event density (although it
can be argued that this gives a distorted view of history, and appropriate cues
should certainly be provided to make the changes in rate explicit). More limited interaction
is available in the Shilla Dynasty animation (Figure 3) generated with TimeMap TMView and
Macromedia Flash.

The logical conclusion of building interactivity into
animation products is to build animation into interactive GIS. Monmonier (1996,
180) uses the term graphic script to describe a script which acts as a
proxy for the user, generating a sequence of views in real time. Mainstream GIS
products provide script languages which can be used in this way, and are moving
towards the integration of animation with GIS functionality. Many can now
generate simple animations with limited interactivity e.g. of GPS tracking data
or 2.5-D flyovers. It is only a matter of time before the generation of
animations based on simulation models or repeated analyses through time become
core functionality.